This invention relates to liquid metal cooled fast breeder nuclear reactors.
In a liquid metal-cooled fast breeder nuclear reactor, the fuel assembly comprises a multiplicity of slender fuel elements or pins over which liquid metal coolant such as sodium is flowed. For convenience, the fuel assembly is divided into a plurality of sub-assemblies, each comprising a plurality of fuel pins enclosed by a tubular wrapper and having a lifting head. The sub-assemblies are positioned in side-by-side array and each one is located in cantilever manner by a lower end spike which is plugged into a fuel assembly supporting structure. The sub-assemblies in the central region of the fuel assembly mainly comprise fissile material whilst the sub-assemblies in the surrounding outer region comprise breeder material. During operation of the nuclear reactor the fertile material captures neutrons emitted by fission in the fissile material to produce further fissile material. As irradiation of the fuel assembly progresses and the fissile content of the breeder material increases, fission of some of the newly formed fissile material takes place so that the power output of the breeder sub-assemblies and, therefore, the coolant temperature, progressively increase. Increased and varying temperatures of coolant streams flowing from the fuel assembly give rise to a condition known in the fast reactor art as thermal striping, a condition which, because of rapid temperature fluctuations, gives rise to cracking in reactor structure material.
An object of the invention is to provide a liquid metal cooled fast breeder nuclear reactor having means which seeks to maintain the outlet coolant temperature of at least some of the breeder sub-assemblies substantially constant throughout the life of the fuel assembly.